It is a widespread custom to endow the surfaces of photographic imaging materials with a certain roughness by means of special measures during production, in order to improve the physical properties that are important during use of the materials, such as. e.g., their tendency to electrostatic charge, their tendency to adhere to other smooth surfaces, their sensitivity to be scratched by dust particles, and their ability during vacuum frame copying to position themselves on the copy materials adequately, quickly and without forming Newton's rings. Although these measures do not always lead to a noticeable decrease in surface gloss, they are generally grouped under the term "matting".
In practical terms, the most important matting measure is the addition of finely divided solids to one or more of the coating solutions used during the production of the imaging materials.
A selection of suitable materials is listed in Research Disclosure 176 043 (December. 1978). Since the addition of matting agents as a rule not only improves the desired properties but also impairs others such as. e.g., clarity or slip, the quantity of the agents must be determined by a compromise based on the application of the imaging material.
If the imaging material is matted on a surface that lies on the same side of the layer support as a light-sensitive layer, a phenomenon called the "starry night" or "pinhole" effect is frequently observed, especially when the light-sensitive layer and the layer containing the matting agent are applied simultaneously. This causes pinhole-like light-permeable dots at the image-forming areas or points of the reproduction after exposure and processing. This effect leads, on the one hand, to a reduction in the average optical density over the imaging range, and on the other hand, to the falsification or destruction of fine structures such as, e.g., half-tone dots or lines of a mask, and is therefore extremely undesirable. It is assumed that this is caused by agglomerates or individual particularly large particles of the generally polydisperse matting agent, which displace the light-sensitive layer when the layer is still deformable during coating and drying.
U.S. Pat. No. 4,172.731 teaches that polymer particles whose color and light absorption is matched to the color of the image in the processed imaging material by means of incorporated pigments, can be used as matting agents. For black and white imaging with silver halide emulsions, for example, polymethyl methacrylate particles that contain carbon black as a pigment, are suggested. It is true that this avoids the starry night effect. The colored particles still present also at the clear points of the processed imaging material, however, cause a light absorption that appears as fog density. Thus, the quantity of matting agent to be added can not be determined by the desired physical properties, but is limited by the intended use of the imaging material.
Photographic imaging materials are known from U.S. Pat. No. 3,523.022 that contain spherical microcrystals of silver halide of between 0.5 and 10 .mu.m in size, as matting agents in their protective layer. These microcrystals are made insensitive to light for this purpose, however. Since they are used in exactly the same way as other matting agents such as, e.g., polymethyl methacrylate spheres or silicon dioxide particles, the starry night effect can not be avoided by the teaching given in this patent.
An object of the invention is to give matted photographic imaging materials that do not exhibit a starry night effect and in which any required degree of matting can be established without impairing the photographic properties.